On 08/18/2014 05:44 AM, Roland Scheidegger wrote: > Am 16.08.2014 02:12, schrieb Connor Abbott: >> I know what you might be thinking right now. "Wait, *another* IR? Don't >> we already have like 5 of those, not counting all the driver-specific >> ones? Isn't this stuff complicated enough already?" Well, there are some >> pretty good reasons to start afresh (again...). In the years we've been >> using GLSL IR, we've come to realize that, in fact, it's not what we >> want *at all* to do optimizations on. Ian has done a talk at FOSDEM that >> highlights some of the problems they've run into: >> >> https://urldefense.proofpoint.com/v1/url?u=https://video.fosdem.org/2014/H1301_Cornil/Saturday/Three_Years_Experience_with_a_Treelike_Shader_IR.webm&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=F4msKE2WxRzA%2BwN%2B25muztFm5TSPwE8HKJfWfR2NgfY%3D%0A&m=iXhCeAYmidPDc1lFo757Cc9V0PvWAN4n3X%2Fw%2B%2F7Lx%2Fs%3D%0A&s=f103fb26bf53eee64318a490517d1ee9ab88ecd29fcdbe49d54b5a27e7581c2e >> >> But here's the summary: >> >> * GLSL IR is way too much of a memory hog, since it has to make a new >> variable for each temporary the compiler creates and then each time you >> want to dereference that temporary you need to create an >> ir_dereference_variable that points to it which is also very >> cache-unfriendly ("downright cache-mean!"). >> >> * The expression trees were originally added so that we could do >> pattern matching to automatically optimize things, but this turned out >> to be both very difficult to do and not very helpful. Instead, all it >> does is add more complexity to the IR without much benefit - with SSA or >> having proper use-def chains, we could get back what the trees give us >> while also being able to do lots more optimizations. >> >> * We don't have the concept of basic blocks in GLSL IR, which makes a >> lot of optimizations harder because they were originally designed with >> basic blocks in mind - take, for example, my SSA series. I had to map a >> whole lot of concepts that were based on the control flow graph to this >> tree of statements that GLSL IR uses, and the end result wound up >> looking nothing at all like the original paper. This problem gets even >> worse for things like e.g. Global Code Motion that depend upon having >> the dominance tree. >> >> I originally wanted to modify GLSL IR to fix these problems by adding >> new instruction types that would address these issues and then >> converting back and forth between the old and the new form, but I >> realized that fixing all the problems would basically mean a complete >> rewrite - and if that's the case, then why don't we start from scratch? >> So I took Ken's suggestions and started designing, and then at Intel >> over the summer started implementing, a completely new IR which I call >> NIR that's at a lower level than GLSL IR, but still high-level enough to >> be mostly device-independant (different drivers may have different >> passes and different ways of lowering e.g. matrix multiplies) so that >> we can do generic optimizations on it. Having support for SSA from the >> beginning was also a must, because lots of optimisations that we really >> want for cleaning up DX9-translated games are either a lot easier in or >> made possible by SSA. I also made the decision for it to be typeless, >> because that's what the cool kids are all doing :) and for a >> lower-level, flat IR it seemed like the thing to do (it could have gone >> either way, though). So the key design points of NIR (pronounced either >> like "near" as in "NIR is near!" or to rhyme with "burr") are: >> >> * It's flat (no expression trees) >> >> * It's typeless >> >> * Modifiers (abs, negate, saturate), swizzles, and write masks are part >> of ALU instructions >> >> * It includes enough GLSL-like things (variables that you can load from >> or store to, function calls) to be hardware-agnostic (although we don't >> have a way to represent matrix multiplies right now, but that could >> easily be added) to be able to do optimizations at a high level, while >> having lowering passes that convert variables to registers and >> input/output/uniform loads/stores that will open up more opportunities >> for optimization and save memory while being more hardware-specific. >> >> * Control flow consists of a tree of if statements and loops, like in >> GLSL IR, except the leaves of the tree are now basic blocks instead of >> instructions. Also, each basic block keeps track of its successors and >> predecessors, so the control flow graph is explicit in the IR. >> >> * SSA is natively supported, and SSA uses point directly to the SSA >> definition, which means that the use-def chains are always there, and >> def-use chains are kept by tracking the set of all uses for each >> definition. >> >> * It's written in C. >> >> (see the README in patch 3 and nir.h in patch 4 for more details) >> >> Some things that are missing or could be improved: >> >> * There's currently no alias tracking for inputs, outputs, and uniforms. >> This is especially important for uniforms because we don't pack them >> like we pack inputs and outputs. >> >> * We need a way to represent matrix multiplies so that we can do >> matrix-flipping optimizations in NIR (currently GLSL IR does this for >> us). >> >> * I'm not entirely happy about how we represent loads and stores in the >> IR. Right now, they're intrinsics, but that means we need a different >> intrinsic for each size and combination of arguments (indirect vs. not >> indirect, etc.) and we might run into a combinatorial explosion problem >> in the future, so we might need to make separate load/store instructions >> like what I did for textures. >> >> * Right now, we only have a pass that lowers variables for scalar >> backends. We need to write a similar pass for vector backends that uses >> std140 packing or something similar, as well as porting >> lower_ubo_reference to NIR and changing it to output offsets in the >> hardware-native units instead of bytes. >> >> * We'll need to write a pass that splits up vector expressions for >> scalar backends. > > Interesting. I think conceptually this makes sense (I'm far from an > expert in that area though), though I wonder if we actually even should > have our own IR? GL NG will specify a common shading language > intermediate representation, and I suspect there'd be benefits if we'd > just use that? Obviously I don't have any idea how that's going to look > like but maybe it will be just like SPIR (which is llvm ir essentially)? > Granted a lot of stuff in your isa is conceptually similar (such as > being based around basic blocks).
It is intended to be an interchange language, nothing more. If I had to guess, I'd say that every driver will read in the GL binary, convert it to its own internal representation, and go about its business. > Roland > > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > http://lists.freedesktop.org/mailman/listinfo/mesa-dev > _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
